This invention relates to reinforced composite materials having a metal matrix. In a preferred embodiment, this invention relates to a method of producing a ceramic-reinforced composite material employing an aluminum alloy as the matrix metal and alumina as a reinforcement material.
Recent technological advances, particularly in the automotive and aerospace industries, have seen the increased need for stronger, lighter, and more durable materials. To meet this demand, materials formulators and producers have directed their attention towards improving the physical properties and ease of manufacture of presently known composite materials.
One composite material that recently has received increased attention is the metal-matrix composite material. In a typical metal-matrix composite, a mass of reinforcing material, such as a ceramic is dispersed within a matrix of metal. For applications requiring high material strength-to-weight ratios, aluminum alloys have been a popular matrix metal.
The integrity and strength of metal-matrix composite materials, however, is limited by an inability to completely bond a mass of reinforcing material to a matrix metal. In instances where bonding is not substantially complete, a decrease in strength, particularly fatigue strength, of the material has been observed. Therefore, it is important to produce a metal-matrix composite material having optimal adhesion between the reinforcing material and metal matrix.
At present, two basic processes and variations thereof are popularly employed to produce metal-matrix composites having ceramic reinforcement material contained in a matrix of aluminum alloy.
The first method, referred to commonly as the "squeeze" method, involves the application of high-pressures to assure infiltration of matrix metal into a mass of reinforcing fibers. See U.S. Pat. Nos. 4,492,265 and 4,450,207. According to these methods, a mass of reinforcing fibers is placed in the cavity of a casting mold and heated. Molten aluminum alloy is then poured into the mold cavity. A relatively high-pressure (in the range of fifteen to thirty-six thousand pounds per square inch) is then applied to the molten metal by a plunger. Upon application of pressure (i.e., squeezing) the molten matrix metal infiltrates the voids in the mass of reinforcing fibers. The elevated pressure is maintained until the aluminum matrix metal solidifies.
This process has certain drawbacks that significantly limit its usefulness. For example, the process is limited in practice to the production of smaller parts. This is due to the tremendous expense and high "squeezing" loads necessary to produce the material. Also, many small and/or fragile reinforcing-fiber preforms cannot withstand the high pressures necessary to practice this process. Thus, fracture and/or displacement of the ceramic reinforcement may frequently occur during this manufacturing process. Further, when extreme caution is not exercised when placing the preform in the mold, fibers from the mass of reinforcing fibers tend to get caught at the parting surface of the molds and thereby prevent a complete closure of the mold. In turn this causes undesireable leaks in the mold and also decreases the yield of successful castings. Finally, even though complete infiltration may be obtained using this process, often there is limited chemical (metallurgical) bonding between the matrix and reinforcements. As is known, mettallurgical bonding is a necessary requirement for maximum mechanical properties.
In a second method, a mass of reinforcing material, such as alumina fibers, is fitted into a stainless steel mold. One end of the mold is dipped into molten aluminum alloy. The pressure at the other end is then reduced creating a suction effect that causes the aluminum alloy to be "sucked up" or to migrate into the mass of fibers, thereby infiltrating the mass of fibers. A critical requirement for successful performance of this process is the use of an aluminum alloy containing a wetting agent of about two to three percent lithium. A wetting agent, such as lithium, is added to materials, such as aluminum, to promote the physical process of wetting, i.e., the process of establishing physical contact between a liquid and a solid. Once contact is established, spreading of the liquid to cover the solid surface can occur by virtue of capillary forces. Hence, the presence of a lithium wetting agent insures that the mass of reinforcing material is substantially infiltrated and bonded to the aluminum alloy.
This process also has significant inherent limitations. That is, the use of costly lithium and rigid metal molds makes this process very expensive. Further, this process does not lend itself readily to the manufacture of parts having anything other than a simple geometric shape. Another negative associated with this process is that rapid cooling techniques, employed of necessity to limit reaction between lithium and the reinforcing fibers, further adds to the cost of the process.
Other less popular processes are known that produce a metal-matrix composite material. One such method involves coating reinforcing fibers with various metals to promote wetting prior to casting the aluminum alloy. For instance, a coating of tin or silver has been applied to alumina just prior to pouring molten aluminum alloy. Or, in a variation of this process, fibers are coated with aluminum alloy and then sintered together, or hot-pressed. The additional coating steps, however, are very costly.
Another known method of producing ceramic-reinforced metal composite materials is a powder metallurgy method. This method can be used to obtain good distribution of reinforcement materials within a metal matrix. However, the requirements of powder metallurgy methods generally make this process unacceptable for continuous fiber or rigid preform composites.
By way of summary, the methods of the present invention relate to the discovery that by heating a molten aluminum alloy and a casting mold to a certain temperature range prior to pouring the molten alloy, the infiltration of the alloy into a mass of reinforcing material is dramatically improved without the necessity of applying an external pressure to force infiltration, and without employing an expensive wetting agent. The methods of the present invention include the steps of heating a molten matrix metal alloy to a temperature substantially greater than the melting point of the alloy. The molten metal is poured into a mold containing a mass of ceramic reinforcing materials. The metal is allowed to solidify.
Accordingly, many of the problems associated with currently available methods used to form metal matrix composites can be overcome by proper control over the casting temperature of both the mold and liquid matrix metal. Because no expensive wetting agents are necessary, and no heavy and excessively large equipment is necessary, complicated shapes can be dependably formed at relatively low cost according to the present invention. Parts produced by the present invention exhibit the desired properties of metal-matrix composites, such as high strength-to-weight ratio, high compressive fatigue strength and relatively good operating characteristics at temperatures significantly above the unreinforced matrix alloy, i.e. in the range of about 300.degree. F. to about 600.degree. F. for aluminum alloys.
The present invention also relates to objects produced by this process.